In controlling the output power level of a high frequency transmitter, it is often desirable to achieve discrete output power levels on a selectable basis. In achieving such selectability, it is necessary that the switching apparatus consume minimum power; that it provide phase-balanced power control at each selection level; and that it employ the minimum amount of circuitry to accomplish the selection function.
One power switching system which is capable of such selective control of output power is shown in FIG. 1. The input from a transmitter is applied via line 10 to a four-arm, switched, power splitting network. The input signal is applied through a conventional Wilkinson splitter which comprises four quarter-wavelength transmission lines 14, 16, 18 and 20, all of which are coupled together through isolation resistors 22, 24, 26 and 28. The Wilkinson circuit functions as an impedance matcher and four-way power splitter. The output from each arm of the Wilkinson splitter is applied to a switching circuit which controls the application of the input signal to a connected RF amplifier (i.e., RF amplifiers 50, 52, 54 and 56). The outputs from the aforementioned amplifiers feed through a Wilkinson combiner 58 to output line 60. One switching circuit 30 is shown in detail and switching circuits 32, 34, and 36 are identical.
Switching circuit 30 comprises two PIN diodes 40 and 42 which are connected via quarter-wavelength transmission lines 44 and 46 respectively, to input line 48 of switch circuit 30. Quarter-wavelength transmission line 44 has one end connected to a terminating resistor 62 whose value is equal to the input impedance of amplifier 50. PIN diode 40 shunts resistor 62 to ground when the diode is in its highly conductive state. As is well known, PIN diodes appear as an RF short circuit when an appropriate DC bias is applied. In FIG. 1 bias source V is connected to diode 40 via coil 64 and switch 66. It should be understood that the circuit for applying a DC bias to PIN diodes 40 and 42 are shown schematically and will generally incorporate semiconductor switching circuits for the appropriate switching function. A similar circuit controls the conduction state of PIN diode 42 via switch 68.
The conductivity states of switches 66 and 68 are controlled by power control circuit 70 (as are the equivalent switching functions in switches 32, 34 and 36). Through appropriate control of the switching states of each of switches 30, 32, 34 and 36, power control 70 can cause the output voltage appearing on lines 60 to vary in level over a number of discrete steps (e.g., 25%, 50%, 75%, 100%, etc.).
The operation of each switching circuit 30, 32, 34 and 36 occurs in the following manner. If it is assumed that power is to pass through switch 30 to amplifier 50, then switch 66 is closed while switch 68 is opened. This results in PIN diode 40 being conductive and PIN diode 42 non-conductive. The conduction of PIN diode 40 short-circuits to ground the lower end of quarter-wavelength line 44, causing it to appear as an infinite impedance at point 72. The non-conductive state of PIN diode 42 allows quarter-wavelength line 46 to appear transparent and to reflect back to its input, the input impedance of amplifier 50. The presence of switching circuit 30 and its series connected transmission line 46 can results in a nominal 0.5 db loss between input line 48 and amplifier 50.
When it is desired to prevent the input signal from arriving at amplifier 50, switch 66 is opened and switched 68 closed, thereby rendering PIN diodes 40 and 42 non-conductive and conductive respectively. Thus, transmission line 44 reflects the impedance of resistor 62 at point 72 and transmission line 46 appears as an infinite impedance. Thus, the input signal still sees the identical input impedance on input line 48 as it sees at the inputs to each of the other switches 32, 34, 36 but no signal reaches amplifier 50. In this manner, balanced feed of the input signal continues while the switching function occurs.
The essential problems with this network arise from the complex circuitry and added losses which result from series connected switches 30, 32, 34 and 36.
Accordingly, it is an object of this invention to provide a multi-path power splitter which provides minimal feed-through loss.
It is still another object of this invention to provide a switched, RF power splitter wherein the number of components are minimized.